1. Field of the Invention
The present invention relates to the field of wireless communications.
2. Description of Related Art
In a spread spectrum communication system, such as the Code Division Multiple Access (CDMA) system specified in the IS-95 standard adopted by the U.S. Telecommunication Industry Association (TIA), a plurality of communication channels share the same radio frequency (RF) band, and are differentiated by unique codes. Each information signal to be transmitted is combined with an assigned code so that the signal appears as noise to a receiver which does not perform a corresponding de-spreading operation. Thus, in contrast to Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) techniques, which provide service to a plurality of mobiles using a single radio frequency (RF) band by assigning different time slots to mobiles and subdividing an RF band into a plurality of sub-bands respectively, the number of mobiles that a single cell/sector of a CDMA system can support at one time is not fixed, and instead is generally limited only by the degradation of service quality caused by interference from a other mobiles in the same or adjacent cells/sectors.
To increase network capacity, CDMA system architectures utilize reverse link (mobile to base station) transit power control techniques to adaptively set the transmit power of each mobile being served to the minimum level needed to maintain adequate performance. Such power control techniques include two main operations: (1) reverse inner loop power control (RILPC)—in which power adjustment commands are generated based on a comparison of reverse link call quality (typically represented as the ratio of energy per bit, Eb, to interference, No) for each mobile being served and a target quality value; and (2) reverse outer loop power control (ROLPC)—in which the target quality value for each served mobile is adjusted to maintain acceptable frame errors rates. More specifically, the base station continuously monitors reverse link Eb/No for each mobile being served and, in accordance with RILPC, generates either a power up-adjust or down-adjust command at predetermined intervals, typically every 1.25 milliseconds, depending on whether reverse link Eb/No is greater than a target Eb/No value assigned to the mobile (indicating acceptable call quality) or less than the target Eb/No value (indicating inadequate call quality). For ROLPC, the base station increases the target Eb/No for a corresponding mobile when a frame error is received (i.e., an erasure frame) to ensure an acceptable frame error rate for the corresponding mobile. If a non-erasure frame is received, the base station lowers the target Eb/No. This process of adjusting target Eb/No levels for each served mobile occurs once every frame, e.g., every 20 milliseconds, and attempts to maintain an acceptable erasure rate for served mobiles while constraining reverse link transmit power on a per call or individual mobile basis (i.e., in a distributed manner).
At certain load levels, the CDMA system may experience abrupt changes in power received at a base station, for example caused by a mobile which does not comply with transmit specifications or when a served mobile comes out of a fade. As another example, the base station will issue a large number of power up-adjust commands under extremely heavy loads, thereby resulting in a sharp increase in interference at the base station. Such a sharp increase in interference will lead to an even greater number of power up-adjust commands. Because many mobiles, particularly those at cell/sector boundaries, will not be able to transmit at the power level needed to overcome the resulting rise in interference, calls may be dropped if the situation persists. Because current reverse link power control techniques are designed to work on a per call or individual mobile basis in a distributed manner, without considering the impact on resulting overall system performance, current power control algorithms do not address the above-described situation.